Exposed wood end grain is like a bundle of tiny straws placed on end. When water comes in contact with the end grain, it tends to be drawn up inside these little straws through capillary action (where the surface tension of water allows it to bridge gaps between objects that are close together). This is bad for the longevity of wood, of course, so we do everything possible to prevent it.
For example, we like to add a layer of self-adhering flashing tape (such as Grace’s Vycor window tape, though there are other brands that will work just as well) to the bottom of our support posts before installation, which separates the bottom of the post from the surface that it is bearing on. In the job shown here (see photo, above), we wrapped 8x8 cedar posts with 6-inch-wide tape, overlapping it so the tape extended up all four sides of the post.
When the post’s base is exposed, we use a square to clean up the tape so it’s even and looks good (see photo, above). After trimming the tape to fit, we go over the seams again to make sure they are all tightly sealed.